<<I haven't heard of any heavy a/c airport where arriving a/c are cleared to land with departing a/c still on the roll, maybe someone could comment?>>
There is such a procedure, certainly at Heathrow - "After the departing cleared to land", providing certain minima are met. I've been away from there too long to remember the precise details but it goes something like.. the departure must either be airborne and ???m down the runway or at least 2500m down the runway. Hopefully one of the EGLL TWR boys will give you the correct gen.. I seem to recall that it is technically legit for both a/c to be on the concrete at the same time - it's the only way you can run a busy single runway airfield. So far as I can recall, it is not a requirement to provide VORTEX separation between aircraft in those circumstances.